Method of smoothing



Nov. 3, 1970 H. P. s. BORGH METHOD OF SMOOTHING Filed March 13, 1967FIG. 70

United States Patent 3,537,216 METHOD OF SMOOTHING Hans P. S. Borgh,Kungalv, Sweden, assignor to Norton Abrasives Limited, Welwyn Garden,England Filed Mar. 13, 1967, Ser. No. 622,638 Claims priority,application Great Britain, Mar. 11, 1966, 10,908/ 66 Int. Cl. B241) ]/00US. Cl. 51-327 6 Claims ABSTRACT OF THE DISCLOSURE Smoothing of aworkpiece is effected by contacting the workpiece with a rotatinggrinding wheel having areas of differing friability while effectingrelative movement between the grinding wheel and the workpiece, thismovement including a component of movement parallel to the axis of thewheel. The surface of the workpiece is contacted with an area of higherfriability after being contacted with an area of lower friability whilethe area of lower friability is still in contact with the workpiece, theareas of lower and higher friability being located at different radialdistances from the wheel axis.

The present invention relates to a method of grinding a surface of aworkpiece to remove blemishes and to produce a relatively smoothsurface.

The invention is especially concerned with the grinding of relativelylarge workpieces. It is applicable, for example, to the grinding ofsteel slabs and billets prior to rolling of such workpieces into sheetform. Such a grinding operation is commonly known as snagging orfettling and involves removal of a comparatively large amount ofmaterial in order to obtain a surface free fiom blemishes or defectswhich otherwise may remain pres ent throughout the rolling process andspoil the final sheet or strip.

In grinding processes of this nature with a grinding wheel of a giventype of abrasive and given grade hardness, it has been shown thatgrinding efficiency (expressed as a ratio of total metal removed towheel wear) increases as the average particle size is increased and thisis more particularly observed when the type of abrasive has a highresistance to fracture. However, it is also observed that the surfacefinish increases in roughness as the particle size is increased and as,in some cases a relatively fine surface finish is required, it has notalways been possible to use a composition giving the highest grindingefficiency. Such coarse surfaces could naturally be reground with agrinding wheel of finer particle size but this would be an even moreexpensive procedure than that if initially using a finer particle sizegrinding wheel of somewhat lower grinding efiiciency.

Snagging operations are often performed using swing frame grinders onwhich the wheel may be moved in any plane or by off-hand methods on afloor-stand machine in which the article to be ground may be moved inany plane. We have observed that, in both cases, the major grinding workappears to be performed by the outer portions or edges of the grindingwheel and the wheel develops a convex grinding face.

Another very important type of machine is the automatic or mechanicalslab and billet snagging machine in which the wheel is maintainedsubstantially perpendicular to the relatively uneven slab or billetsurface, is maintained in high pressure contact with the workpiece by asuitable source of fluid pressure, is traversed in an axial directionalong the workpiece (or the workpiece similarly traversed relative tothe stationary wheel head) and, at the end of the traverse in onedirection the wheel ice head or workpiece is suitably cross indexed andthe traverse repeated in the reverse direction. Again we have observedthat the major work is performed with the outer portions or edges of thegrinding wheel, the traverse in one direction first causing the wheel towear more rapidly from the leading edge and from the op posite edge onthe reverse traverse, again leading to the formation of a substantiallyconvex wheel face.

The present invention provides a method of smoothing a workpiececomprising contacting said workpiece with the peripheral surface of arotating grinding wheel, said peripheral surface including a firstsurface area of relatively high grinding efficiency and a second surfacearea of relatively low grinding efficiency, said first surface areabeing located at a different radial distance from the wheel axis thansaid second surface area, while effecting relative movement between saidworkpiece and said wheel such that said workpiece is contacted by saidsecond surface area after being contacted by said first surface area,said relative movement including a component of movement parallel to theaxis of rotation of said wheel, whereby to impart to said workpiece asurface finish superior to that which would be achieved by contactingthe workpiece solely with said first area while achieving a grindingefficiency greater than that which would be achieved by contacting theworkpiece solely with said second area.

The invention also provides a grinding wheel having a peripheral surfaceincluding a first area of relatively high grinding efficiency and asecond area of relatively low grinding efficiency, said first area beinglocated at a different radial distance from the wheel axis than saidsecond area.

The invention is especially applicable to snagging or fettlingoperations in which a grinding wheel is brought into contact with theworkpiece while effecting relative movement between the grinding wheeland the workpiece with a component of movement parallel to the grindingwheel axis, the grinding wheel usually being moved over the workpiece ina path substantially parallel to the grinding wheel axis. By contrastwith known operations of this type wherein the grinding wheel has aperipheral surface of uniform grinding efliciency the present inventioninvolves the use of a grinding wheel having a peripheral surface havingat least one area of higher grinding efficiency than another areathereof.

Thus, for example, when the invention is applied in association with aknown snagging or fettling operation where the grinding wheel is movedforwardly and back- Wardly in reciprocating fashion over the surface ofthe workpiece, the grinding wheel employed may be so formed as to have ahigher grinding efficiency at each side of its peripheral surface thanat the centre of the peripheral surface. The simplest constructionalform of such a wheel is represented by a wheel formed of two outerlayers of high grinding efliciency and an inner layer of lower grindingefliciency sandwiched between the two outer layers.

When a cylindrical wheel of this type is used in a snagging or fettlingoperation wherein there is reciprocating movement between the wheel andthe workpiece it is found that both outer edges of the wheel peripheryacquire a camber so that the peripheral surface of the wheel becomessmoothly convex. Continued grinding of the workpiece with the camberedwheel then results in the production of a finer surface finish than thatwhich would be attained by'using a wheel made solely of the material ofthe outer layers while achieving a grinding efficiency higher than thatwhich would be achieved by using a wheel made solely of the material ofthe inner layer.

It should be noted that the wheel may have difierent structures fromthat just described. It may for example be so formed that there is acontinuous gradation in composition and grinding efliciency from oneperipheral edge of the wheel to the opposite edge of the wheel with thisefficiency being at a minimum at the centre line of the peripheral face.However, wheels of this type are relatively difiicult to manufacture andit is generally more practical to provide the desired gradation ofgrinding efliciency across the peripheral face of the wheel byassembling a number of layers of differing grinding efiiciencies, as inthe case of the three layer wheel described above. Clearly the number oflayers can be increased beyond three. For example, a five layer wheelmay be formed by sandwiching an inner layer of low grinding efficiencybetween two intermediate layers of higher grinding efiiciency and thenapplying two outermost layers of still higher grinding efiiciency.

These various types of wheels all become cambered at both peripheraledges when used in a reciprocating type of fettlin-g or snaggingoperation. As a result the workpiece is always first contacted by anarea of higher grinding elficiency and is last in contact with an areaof lower grinding efficiency irrespective of the direction of relativemovement between the wheel and the workpiece. The presence of thecambered edges on the wheel is an important feature of the method of theinvention and in some cases itmay. be desired to form the wheels withsuch cambers during their manufacture rather than form the cambers atthe start of the grinding operation.

Snagging and fettling operations where the grinding wheel is movedacross the surface of the workpiece in one direction only, rather thanbeing reciprocated over it, are uncommon. However, in such a case themethod of the invention would involve the use of a wheel one peripheraledge of which Was of higher grinding efiiciency than the otherperipheral edge. The wheel could simply consist of two layers, one ofhigher grinding efliciency than the other or anumber of layers could beused arranged in order of increasing grinding efliciency. Alternativelythe composition of a one piece wheel might be so varied from oneperipheral edge to the other as to produce a continuously increasinggrinding efiiciency across the peripheral face. Whatever the particularstructure chosen the grinding operation is so conducted that theperipheral edge of higher grinding efiiciency first contacts theworkpiece. This edge acquires a camber (or may be so preformed) and theresults obtained are then similar to those described in respect of thereciprocating type of snagging or fettling operation, i.e. a goodsurface finish is obtained while attaining a grinding efficiency notnormally feasible in conjunction with such a finish.

The grinding wheels described above for use in the method of theinvention are characterized by having the first area of relatively highgrinding efl-iciency located at a lesser radical distance from the wheelaxis than is the second area of relatively low grinding efliciency. Whenusing such wheels it is possible to operate by movmg the wheel over theworkpiece with its median plane substantially at right angles to thesurface of the workpiece. Alternatively, the wheel can be moved over theworkpiece with its median plane at an acute angle to the surface oftheworkpiece in the direction of the relative "movement between thewheel and the workpiece. It is only when the wheel is moved over theworkpiece with its .median plane at an obtuse angle to the direction ofmovement. Wheels of this second type are characterized by having thefirst area of relatively high grinding efficiency located at a greaterradial distance from the wheels axis than in said second area. It willbe appreciated that this second type of grinding wheel is completelyopposite in construction to the first type of grinding wheel.

Thus, a grinding wheel of the second type may be so formed as to have alower grinding efliciency at each side of its peripheral surface than atthe centre of the peripheral surface. The simplest constructional formof such a wheel is represented by a wheel formed of two outer layers oflow grinding elficiency and an inner layer of higher grinding efiiciencysandwiched between the two outer layers.

When a cylindrical wheel of this type is used in a snagging or fettlingoperation wherein there is reciprocating movement between the wheel andthe workpiece it is found, as in the case of the wheels of the firsttype, that the peripheral surface of the wheel becomes smoothly convex.Provided that the obtuse angle between the wheel and the workpiecementioned above is maintained it is found that the workpiece acquires afiner surface finish than that which would be attained by using a wheelmade solely of the material of the inner layer while achieving agrinding efiiciency higher than that which would be achieved by using awheel made solely of the material of the outer layers.

As in the case of the Wheels of the first type it is possible to formthe wheels of the second type with various structures. Thus, the wheelmay have a continuous gradation in composition and grindingefiiciencyfrom one peripheral edge of the wheel to the opposite peripheral edgewith this efficiency being at a maximum at the centre line of theperipheral face. Alternatively, the wheels of the second type mayinclude more layers than three with the middle layer having a highgrinding efficiency and outer layers of grinding efliciencies whichdecrease towards the edges of the Wheel. All of these wheels acquire aconvex peripheral face when used in the method of the invention if theyhave not been preformed with such a face.

The invention will be described by Way of illustration and withoutlimitation with reference to the accompanying drawings in which:

FIGS. 1, 2 and 3 show grinding wheels suitable for use in the method ofthe invention, and

FIGS. 1a, 2a and 3a illustrate more clearly the contours of theperipheral surfaces of the wheels of FIGS. 1, 2 and 3.

FIGS. 4, 5 and 6 illustrate different modes of movement of a wheel overa workpiece.

As shown in FIG. 1a wheel 1 includes two outer annular layers 2 of highgrinding efiiciency composed of a relatively coarse abrasive materialand an inner annular layer 3 of lower grinding efliciency composed offiner or more friable abrasive material.

Such a combination in a grinding wheel gave the surprising result that,without showing excessive wear of the inner portion, the wheel gave,when used in a reciprocating type of fettling or snagging operation withthe Wheel moving substantially parallel to its axis of rotation, asurface finish better than that to be expected from the coarse outerlayer composition and, coincidentally, a grinding efiiciency higher thanthat to be expected from the liner or more friable abrasive inner layeror layers.

Such a combination of layers may be moulded as a single wheel by knownmethods, layers may be made separately and cemented together to form asingle wheel,

'or layers may be made as separate wheels and mounted appropriately on asingle spindle on the grinding machine to provide a unitary laminatedstructure of the desired layer distribution.

FIG. 1a illustrates in exaggerated form contour of the peripheralsurface of the wheel of FIG. 1. According to the invention the wheel 1can be formed with such a contour during its manufacture. Alternatively,the wheel can be formed cylindrically; the peripheral edges of the wheelthen become cambered at the start of the fettling or snagging operationso that the peripheral surface of the wheel becomes slightly convex asin FIG. 1a. It will be appreciated that because of this convexity theworkpiece surface is past contacted by the peripheral surface of layer 3in both backward and forward passes of the wheel over the workpiece.

FIG. 2 illustrates another type of wheel suitable for use according tothe invention in a reciprocating type of fettliug r snagging operation.The wheel 4 0f FIG. 2 includes outermost annular layers 5 of highgrinding efficiency, intermediate annular layers 6 of lower grindingefiiciency and a middle annular layer 7 of still lower grindingefiiciency. FIG. 3a shows in exaggerated fashion how the peripheralsurface of the wheel is formed in manufacture or how it becomes convexat the start of the grinding operation so that the peripheral surface oflayer 7 is always last in contact with the workpiece and determines thenature of the finish.

FIG. 3 illustrates a grinding wheel suitable for use according to theinvention in a grinding or fettl'ing operation in which the wheel passesover the surface of the workpiece in only one direction. The wheel 8 iscomposed of an annular layer 9 of high grinding efficiency and anannular layer 10 of lower grinding efficiency and is moved across theworkpiece with the layer 9 providing the leading edge. At the start ofthe grinding operation the peripheral surface of the wheel acquires acamber as indicated in exaggerated fashion in FIG. 3a. Alternatively thewheel may be so preformed. The grinding efficiency is comparable withthat corresponding to use of a wheel made solely of the material oflayer 9 while the surface finish achieved is comparable with thatcorresponding to use of a wheel made solely of the material of layer 10.

FIGS. 4 and 5 illustrate alternative ways in which a wheel of the typeshown in FIGS. 1, 2 or 3 is moved across the surface of a workpiece 11.In each of the figures the wheel moves in the direction indicated by thearrow. In FIG. 4 the wheel 1 is moved with its median plane at rightangles to the surface of the workpiece 11. In FIG. 5 the wheel 1 moveswith its median plane at an acute angle to the surface of the workpiece11 in the direction of movement of the wheel. It will be appreciatedthat whereas in the arrangement of FIG. 4 the wheel 1 can simply bereciprocated without changing its attitude at the end of each pass, inthe arrangement of FIG. 5 the tilt of the wheel 1 must be reversed atthe end of each pass so as to maintain the desired acute angle betweenthe median plane of the wheel and the surface of the workpiece 11.Moreover, in the case of the wheel of FIG. 3 the wheel would have to berotated through 180 at the end of each pass in order that the surface oflower grinding efiiciency is last in contact with the surface of theworkpiece.

The wheels of FIGS. 1, 2 and 3 are all of the type in which the firstarea of relatively high grinding efficiency is located at a lesserradial distance from the wheel axis than the second area. It will bereadily understood from the foregoing disclosure that wheels having thefirst area of relatively high grinding efiiciency located at a greaterradial distance from the wheel axis than a second area of relatively lowgrinding efficiency can be made by simply reversing the construction ofthe wheels of FIGS. 1, 2 and 3. Thus, the wheel of FIG. 1 can bemodified by having the layers 2 of low grinding efiiciency and the layer3 of high grinding efficiency. Similarly, the wheel of FIG. 2 can bemodified by having the layers 5 of lower grinding efficiency, the layers6 of higher grinding efliciency and the layer 7 of still higher grindingefiiciency. Also, the wheel of FIG. 3 can be modified by having thelayer 9 of lower grinding efficiency and the layer 10 of higher grindingefliciency. It will be appreciated that the wheels may be preformed withthe peripheral edge cambered in the manner described or alternativelymay be caused to acquire such a camber at the beginning of the snaggingor fettling operation.

FIG. 6 illustrates the way in which these modified wheels of the secondtype are moved over the surface of the workpiece. The wheel 12 moveswith its median plane at an obtuse angle to the surface of the workpiecein the direction of movement of the wheel. The object is to ensure thatthe surface of lowest grinding efiiciency is last in contact with theworkpiece. As will be understood, the angle of tilt of the wheel 12 isreversed at the end of each pass when the wheel is used in areciprocating type of operation.

In a particular example a wheel of the type illustrated in FIG; 1 wasconstructed with two outer layers 2 each 25 mm. thick composed of grainsof sintered bauxite, particle size 14 mesh, bonded together by asynthetic resin binder and an inner layer 3 25 mm. thick composed ofgrains of sintered bauxite, particle size 20 mesh, bonded together bythe same resin binder. The layers 2 and 3 were also bonded together bymeans of the same resin binder to form a unitary assembly.

This wheel (referred to as Wheel A below) was used in an automatic slabsnagging machine for smoothing the surfaces of stainless steel slabs,the wheel being moved over the slab in a direction substantiallyparallel to the wheel axis and with its median plane substantially atright angles to the surfaces of the workpiece. Comparative tests werealso carried out using in the same machine two wheels of similardimensions, one 75 mm. thick composed entirely of sintered bauxite ofparticle size 14 mesh (Wheel B) and one 75 mm. thick composed entirelyof sintered bauxite of particle size 20 mesh (Wheel C).

As was to be expected it was found that the grinding efiiciency of WheelB (expressed as ratio of total metal removed from the slab to the lossof material from the wheel) was higher than that of Wheel C but that thesurface finish imparted to the slabwas smoother with Wheel C. With WheelA on the other hand the grinding efficiency was only slightly less thanin the case of Wheel B while the finish was only slightly less smooththan that in the case of Wheel C.

In a further series of experiments wheels were made having layers ofdifferent abrasive materials but of similar particle size and weretested in the same automatic snagging machine on austenitic steelworkpieces and on ferritic steel workpieces. The wheels were all bondedusing the same synthetic resin binder and all of the abrasive particleshad a particle size of 14 mesh. The structures of the wheels were asfollows:

The grinding efficiencies of these wheels on austenitic steel andferritic stainless steel were as follows, the efiiciency being expressedin each case as the ratio between kilograms of metal removed to thevolume of wheel wear in cubic decimetres.

Wheel D Wheel E Wheel F Wheel G Austenitic steel- 14. 5 35. 3 38. 9 44.7 Ferritie steel 14. 4 28. 9 30. 8 35. 9

Wheel E produced in both cases a surface finish almost as good as thatproduced by Wheel D. Wheel F produced in both cases a surface finishinferior to that produced 7 y by Wheel E but considerably superior tothat produced by Wheel G.

The invention has been described with reference to particularembodiments thereof and it will be appreciated that it is not limitedthereto. Many variations and modifications are clearly feasible withinthe scope of the invention. As has been described, the wheels used inthe invention may comprise layers of the same abrasive material but ofdifferent particle size or layers of different abrasive materials of thesame particle size It will be understood that'the layers could also beformed of different materials of different particle size. 7

Any of the materials and techniques known in the prior art may beutilised. Sintered bauxite and fused alumina have been specificallymentioned but other abrasive materials can be used. However, sinteredbauxite is a preferred material since .it has such a high impactresistance. The manufacture of the wheels can be undertaken with the aidof such known techniques as hot pressing using resin binders of knowntypes.

In conclusion it may be noted that grinding wheels comprising planarlayers of differing composition of abrasive materials are known but inprevious instances each composition is designed to perform a completegrinding operation upon a section of a given article. Also, in somecases, very thin outer layers of a composition of harder grade than themain body of the wheel may be used to avoidbreakdown or rapid wear ofcorners when .cylindrically grinding -into fillets and similaroperations. 1 l

Additionally, grinding wheels for snaggingor fettling are known havingannular layers increasing in grade hardness from periphery to centre tocompensate for deduction in surface speed as the wheel wears downy.

None of these knowncases are comparable with the present method whereinall layers traverse the whole of the workpiece area almostsimultaneously. Knowledge of these known procedures could not lead to aprediction of the outstanding combination of grinding efliciency andfinish-provided by our invention.

Moreover, the wheels of the prior art have been of a cylindrical nature.by contrast with the wheels of the present invention which have apreipheral surface including a surface. area of relatively high grindingefliciency radially displaced from the wheel axis a different distancethan a surface area'of relatively low grinding efficiency. 7

What I claim is:

1. A method of smoothing a workpiece comprising contracting saidworkpiece with the peripheral surface of a rotating grinding wheel,'saidperipheral surface including a first surface area of relatively lowfriability and a second surface area of relatively high friability, saidfirst surface area being located at a different radial distance from thewheel axis than said second surface area, and effecting relativemovement between said workpiece and said wheel such that the surface ofsaid workpiece is contacted :by said second surface area after beingcontacted by said first surface area and while said first surface areais still in contactwith the surface of said workpiece, said relativemovement including a component of movement parallel to ;the axis ofrotation of said wheel, whereby to impart to said workpiece a surfacefinish superior to that which would be achieved by contacting theworkpiece solely with said first area while achieving a grindingefliciency greater than that which would be achieved by contacting theworkpiece solely with said second area.

2. The method of claim 1 wherein said relative movement is.substantially parallel to said axis of rotation.

3. The method of claim 1 wherein said wheel is moved reciprocatinglyover said workpiece.

4. The method of claim 1 wherein said wheel comprises two outer annularlayers of low friability and an inner annular layer of higher friabilitysandwiched between said outer layers, and said wheel is moved over saidworkpiece with its median plane substantially at right angles to thesurface of the workpiece-in the direction of said movement.

5. The method of claim 1 wherein said wheel comprises two outer annularlayers of low friability and an inner annular layer of higher friabilitysandwiched between said outer layers, and said wheel is moved over saidworkpiece with its median plane at an acute angle to the surface of theworkpiece in the direction of said movement.

6. The method of claim 1 wherein said wheel comprises two outer annularlayers of high friability and an inner annular layer of lower friabilitysandwiched between said outer layers, and said wheel is moved over saidworkpiece with its median plane at an obtuse angle to the surface of theworkpiece in the direction of said movement.

References Cited UNITED STATES PATENTS 1,009,877 11/1911 Winn.

1,616,531 2/1927 King 51-207 2,198,377 4/1940 Dunbar 51-48 3,019,5622/1962 Price 51-3 HAROLD D. WHITEHEAD, Primary Examiner

